Nihilo, also known as Nihilo the Vortical Stampede, is a revenant weapon of war originally created by a self-proclaimed demon king. She is an undead bioweapon, appearing as a girl whose body is stitched together from multiple corpses through forbidden rituals. Her form is augmented with spider-type weaponry and she commands a massive creature known as Helneten the Burial, which she can merge with to control. Nihilo also has the ability to control other living beings, such as forcing an assassin to discard his weapon and then stop breathing, and she can manipulate tendrils from her back to inflict nerve damage or spin webs to puppeteer her enemies. She is capable of spreading lethal viruses and possesses close combat skills that surpass those of seasoned warriors, having single-handedly annihilated an entire regional army.

After the death of her creator, Nihilo was captured and held prisoner by the kingdom of Aureatia. She joined the military operation against the breakaway New Principality of Lithia in exchange for her freedom. Her primary motivation is to gain liberation from captivity, though her status as a manufactured weapon leaves her questioning her own humanity. In her role within the story, she serves as a powerful asset for Aureatia, deployed as a living weapon on the battlefield. Her key relationship is with Hidow, the official who secures her release in return for her cooperation. She faces off against the bandit Dakai the Magpie, who ultimately decapitates her in combat. However, as a revenant, Nihilo is not permanently destroyed; in later events of the light novel series, she is shown to have been reassembled and returns to the narrative. Her development revolves around her transition from a captive tool to an active participant in the conflict, struggling with her own identity and purpose. Her notable abilities include possession and control of other beings, manipulation of Helneten the Burial, and a suite of organic weaponry that makes her a formidable force on the battlefield.